The present invention relates generally to a super-conductive coil in a synchronous rotating machine. More particularly, the present invention relates to a support structure for super-conducting field windings in the rotor of a synchronous machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K., to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature. As a result, no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a super-conducting field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conductive rotors.
In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal and other acceleration forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start-up and shut-down cycles over the course of several years that result in low cycle fatigue loading of the rotor, and apply bending moments that strain the rotor coil. Furthermore, the HTS rotor coil winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature, and notwithstanding occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. They are subjected to centrifugal loading, bending moments, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system. These support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal and other acceleration forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
A challenge to the development of a high temperature super-conducting (HTS) electric machine is maintaining the structural integrity of the super-conducting field coil. Due to the brittle coil, the critical current of a BSCCO based coil is sensitive to the level of mechanical strain in the coil. Accordingly, the mechanical strain into the SC coil should be minimized to maintain the optimal level of critical current.
A robust coil support is needed to minimize the mechanical strain in the SC coil for a rotor of a synchronous machine. By minimizing the coil strain, the coil support ensures that the coil retains its critical current capability. In addition to minimizing coil strain, the coil support should not conduct heat from the rotor to the cryogenic coil.
Developing support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy-to-fabricate components.
A coil support has been developed that has a split clamp that grasps the ends of a race-track SC coil. The clamp provides rigidity and stiffness to the coil, and prevents the coil from bending during centrifugal acceleration. By stiffening the coil, the clamp minimizes strain in the coil and thereby retains the critical current capability of the coil.
The split clamp is a free-floating attachment to the coil, and is not secured to the rotor. Thus, the split clamp may be held at a cryogenic cold temperature along with the coil. The free-floating clamp is thermally isolated from hot structures such as the rotor core and end shaft collar. The free-floating clamp does not require insulation structures to prevent heat from the rotor from conducting through the clamp into the coil.
The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
The coil support system, including the split clamp, is useful in integrating the coil support system with the coil and rotor. In addition, the coil support system facilitates easy pre-assembly of the coil support system, coil and rotor core prior to final rotor assembly. Pre-assembly reduces coil and rotor assembly time, improves coil support quality, and reduces coil assembly variations.
In a first embodiment, the invention is a rotor for a synchronous machine comprising: a rotor core; a super-conducting coil winding extending around at least a portion of the rotor core, the coil winding having a coil end section adjacent an end of the rotor core, and a coil support bracing the end section and being thermally isolated from the rotor core.
In a second embodiment, the invention is a method for supporting a super-conducting coil winding on a rotor core of a synchronous machine comprising the steps of: bracing an end section of the coil winding with an end coil support; assembling the coil winding, end coil support and rotor core; attaching a rotor end shaft to the rotor core; and thermally isolating the coil support from the rotor core and shaft.
In a third embodiment, the invention is a rotor for a synchronous machine comprising: a rotor core having at least one rotor core end orthogonal to a longitudinal axis of the rotor; at least one end shaft attached to the rotor core end; a race-track super-conducting (SC) coil winding extending around the rotor core and having a coil end section adjacent the rotor end; a coil support brace attached to the coil end section and thermally isolated from the rotor core and rotor end shaft.